1. Field of the Invention
The present invention relates to an image reading apparatus for reading an image signal by using a linear image sensor which stores the image signal of one scanning line in a predetermined time period.
2. Description of the Prior Art
An image reading apparatus using an image sensor, e.g., a CCD (Charge Coupled Device) linear image sensor for storing an image signal of one scanning line in the main scanning direction in a predetermined time period is widely used as an image reading apparatus for equipment such as a facsimile or the like. A conventional image reading method used in this kind of image reading apparatus will now be explained with reference to FIGS. 1(a) to 1(f). FIGS. 1(a) to 1(f) are timing charts showing the timing of a signal at the reading time in each section of the image reading apparatus according to a conventional image reading method.
A start pulse shown in FIG. 1(a) is outputted to a linear image sensor at a predetermined period from a control section of the image reading apparatus at the time of reading. Synchronously with this start pulse, the charges corresponding to the amount of light received in the linear image sensor are accumulated, namely, the image signal of one scanning line in the main scanning direction of an original is stored for the storage interval between the start pulse and the next start pulse.
In addition, as shown in FIG. 1(b), synchronously with this start pulse, the image signal stored in the linear image sensor for the storage interval immediately before the output of that start pulse is read out as a turn-out signal for a predetermined period shorter than the storage interval and is led to a gate connected to an output terminal of the image reading apparatus.
When a reading request pulse shown in FIG. 1(c) is applied, a gate signal shown in FIG. 1(d) is outputted from the control section to the gate synchronously with the start pulse immediately after that pulse. Thus, the gate is opened and the turn-out signal of FIG. 1(b) is outputted as an output signal shown in FIG. 1(e).
On the other hand, a pulse of an original drive signal shown in FIG. 1(f) is supplied from the control section to an original drive section synchronously with the start pulse immediately after the reading request pulse, thereby allowing the original to be moved by one scanning line.
In the foregoing conventional reading method, the storage and turn-out of the image signal of one scanning line of the original are carried out synchronously with the start pulse which is generated at a predetermined period; the image signal which is turned out is read out synchronously with the start pulse immediately after the reading request pulse was generated; and the original is moved by one scanning line synchronously with that start pulse.
However, according to such a reading method, there occur cases where the image signal stored when the original is at rest is outputted at the timing of the reading request pulse, and cases where the image signal stored when the original is moving is outputted.
Namely, according to this method, in the case where the reading request pulse is generated in a storage interval over a predetermined period after the immediately preceding reading request pulse, the original is not driven but is at rest during the storage interval after the following reading request pulse is generated, and the image signal stored for this storage interval is outputted synchronously with the next start pulse.
In contrast, in the case where the reading request pulses are respectively generated for two storage intervals that are continuous before and after, the original is driven for the later storage interval synchronously with the start pulse immediately after the preceding reading request pulse, and the image signal stored for this storage interval is outputted in response to the later reading request pulse.
As described above, according to the conventional image reading method, as an image signal which is outputted, there are two kinds of such image signals, viz., those stored when the original is at rest and those stored when the original is being driven. Thus, this causes a variation in scanning line density and the image cannot be read out accurately.